1. Field of the Invention
This invention relates to an optical scanning device, and an optical element for use therein, for scanning an optical record carrier, such as an optical disk, comprising an information layer, the device comprising a radiation source for generating a radiation beam and an objective lens, located in an optical path between the radiation source and the information layer, for converging the radiation beam to a spot on the information layer. In particular, but not exclusively, the invention relates to an optical scanning device including an optical element to compensate for spherical aberrations generated by different optical path lengths (referred to herein as information layer depths) through which the beam travels in an optical disk to reach an information layer in the disk.
2. Description of the Related Art
There is a need for the production of optical record carriers of high capacity. Therefore, optical scanning devices using a relatively short wavelength radiation beam, for example, a radiation beam of 400 nm, and a high numerical aperture (NA) objective lens system, at least 0.7 and, for example, NA=0.85, are desirable. Furthermore, the capacity can be increased by providing a dual layer disk. At the wavelength and NA mentioned, a layer separation of at least 20–30 μm is desirable in order to reduce the coherent cross talk to an acceptable level. Without compensating measures, refocusing from one layer to the other results in spherical aberration, generating a wavefront error of 200–300 mλ root mean square optical path difference (OPD (rms)), which deteriorates the resolution of the optical spot formed.
It is known to mechanically adjust the spacing of the two, or more, lens elements of a compound objective lens, in order to provide spherical aberration compensation. Another method of compensation is by mechanically adjusting the position of a collimator lens with respect to the radiation source, so that the radiation beam impinges on the objective lens as a convergent, or divergent, instead of collimated, beam. Each of these methods compensates spherical aberration generated in the optical system of the scanning device, to at least approximately cancel out that generated in the optical disk being scanned.
However, using mechanical actuators to provide spherical aberration compensation, particularly when a separate mechanical actuator is used to provide focus control, is relatively complex and, therefore, increases the cost of manufacture of the scanning device.
A further known optical scanning device is described in International Patent Application No. WO-A-01/24174, corresponding to U.S. Pat. No. 6,567,365, in which a radiation beam is passed through a twisted nematic (TN) liquid crystal cell which selectively rotates the polarization of incident light by 90°. The beam is then passed, when in a convergent state, through a birefringent plate to produce spherical aberration therein. The birefringent plate produces different amounts of spherical aberration depending on the state of the TN cell, to compensate for the different information layer thicknesses.
European Patent Application No. EP-A-0865037 A1, corresponding to U.S. Pat. No. 6,215,756, and an article in Applied Optics, volume 38 (1999) pp 3778–3786 by R. Katayama, describe a phase structure which is used to make an objective lens designed for scanning DVD record carriers also suitable for scanning CD record carriers. In general, DVD record carriers are designed for being scanned with a radiation beam of a wavelength and numerical aperture different from that used for scanning a previous generation of record carriers, such as CD. The phase structure consists of stepped non-periodic annular zones, such that each zone gives rise to a phase step which is equal to a multiple of 2π for the DVD wavelength (660 nm), so that the phase structure has no effect at this wavelength. For CD read out, however, a different wavelength is used (785 nm). Consequently, the stepped phase profile results now in phase steps which no longer are equal to a multiple of 2π. With a proper design of the steps heights and zone width, the phase introduced by the phase structure in the CD case reduces the wavefront aberration caused by the information layer depth difference to below the diffraction limit. The structure is capable of reducing the wavefront aberration for two discrete wavelengths.
Japanese Patent Publication No. JP-A-2000-163791 describes a diffraction grating for an optical head for an optical scanning device capable of operating at two different wavelengths. Two different radiation sources are used and a grating is located behind a dichroic reflector forming a folding mirror for directing the radiation towards an objective system. The diffraction grating is a periodic structure, i.e., one which regularly repeats across the element. For one wavelength, the dichroic reflector reflects the radiation beam without diffraction, while for the other wavelength, the dichroic reflector transmits the radiation beam and the grating diffracts the radiation. In this way, while the optical axes of the beams emanating from the two radiation sources do not initially coincide, the beams pass coaxially through the objective system. Due to the diffraction, a certain amount of the input light is wasted, which is undesirable.